Aconvergence zone inmeteorology is a region in theatmosphere where twoprevailing flows meet and interact, usually resulting in distinctiveweather conditions.[1]This causes a mass accumulation that eventually leads to a vertical movement and to the formation ofclouds andprecipitation.[1] Large-scale convergence, calledsynoptic-scale convergence, is associated withweather systems such asbaroclinic troughs,low-pressure areas, andcyclones. The large-scale convergence zone formed over the equator, theIntertropical Convergence Zone, has condensed and intensified as a result of the global increase in temperature.[2] Small-scale convergence will give phenomena from isolatedcumulus clouds to large areas ofthunderstorms.
The inverse of convergence is divergence, such as thehorse latitudes.
An example of a convergence zone is theIntertropical Convergence Zone (ITCZ), alow pressure area which girdles theEarth at theEquator.[3] Another example is theSouth Pacific convergence zone that extends from the westernPacific Ocean towardFrench Polynesia.
TheIntertropical Convergence Zone is the result of the northeasterlytrade winds and southeasterly trade winds converging in an area of highlatent heat andlow pressure.[3] As the two trade winds converge, the cool, dry air collects moisture from the warm ocean and rises, contributing to cloud formation and precipitation. The low pressure area that is created by the movement of the trade winds acts as avacuum, drawing in the cooler, dry air from high pressure areas (divergence zones), creating a convection cell commonly known as theHadley Cell.[3]
Sea surface temperature is directly related to the position of the Sun or the location of the "energy flux equator," thus theITCZ shifts corresponding to the seasons.[3] Due to the position of the Sun, the sea surface temperature near the equator (30°S to 30°N), during anequinox, is higher than any other latitudes.[4] During thesummer solstice in theNorthern Hemisphere (June 21), theITCZ is shifted north, following the position of the Sun.[5] TheITCZ is shifted farther south during thewinter solstice (in the Northern Hemisphere), when the solar radiation is focused at 23.5°S.
Convergence zones also occur at a smaller scale. Convergence lines form rows of showers or thunderstorms over a more local area.Sea breezescolliding can trigger development of a convergence line. The heavy rain caused in a short period of time can cause severe flooding.[6]
Some examples are thePuget Sound Convergence Zone which occurs in thePuget Sound region in the U.S. state ofWashington;Mohawk–Hudson convergence in the U.S. state ofNew York; theElsinore Convergence Zone in the U.S. state ofCalifornia; theBrown Willy effect which can be generated when south-westerly winds blow overBodmin Moor inCornwall; and thePembrokeshire Dangler which can form when northerly winds blow down theIrish Sea.Flooding in Boscastle, Cornwall, England in August 2004 was the result of thunderstorms developing on a convergence line.[6]